First of all, let me point out that WordPress is seriously behind when it comes to multilingual and international websites. That being said, several plugins are available to fix that problem. Of all those plugins, that two best solutions are probably WPML and xili-language.
Now which one to go with? From a regular user's perspective, the choice of a plugin is usually pretty simple. It boils down to how easy it is to use and how easy the administration interface is. Fortunately for most users, both WPML and xili-language are a breeze to use.
However, for more experienced users, there's a lot of things happening in the code "behind the scene". The first thing you can notice when comparing WPML and xili-language is that xili-language does not create addition tables in your database as compared to its competitors. xili-language simply makes proper use of the taxonomy tables offered by WordPress. Moreover, it uses .mo and .po files hosted on your server which is the proper way to translate WordPress themes and several web applications. In short: xili-language functions the way WordPress was intended to work.
On top of that, xili-languages comes with two formidable plugins that together create a perfect multilingual package. So if you don't really care about what's happening in your code and only care about how easy your administration interface is, those two extra plugins will definitely make xili-language work for you.
xili-tidy-tags lets you create multiple group of tags. That way, you can have a tag cloud for tags in English, another cloud for French tags, another for Spanish ones, and so on. You can also use the plugin for more than multilingual blogs. Basically, you can create any group of tags you want.
xili-dictionary offers a really simple interface for translating all the strings in your WordPress theme. If your theme is already multilingual ready, such as Fusion or the international version of Kubrick, xili-dictionary will detect it and let you translate every single string in the theme by simply importing all terms from your theme. If you're building a theme from scratch, then it's even easier, you build your theme in the default language you want to use and then use xili-dictionary to translate it.
On top of that, the noticeable features that make xili-language our favourite international plugin are:
-xili-dictionary & xili-tidy-tags
-Detection of the browser's language
-Corresponding posts in other languages
-Addition of a language column when browsing posts list
-Does not interfere with any plugin we tested it with
Finally, our company has been using xili-language since early 2009 for all our multilingual WordPress projects and none of our client ever complained. Plus, the vast majority of our clients are not really technology friendly and they still manage to administrate their own WordPress themselves.